Lithium ion secondary battery

ABSTRACT

The present invention relates to a secondary battery, specifically, a secondary battery having excellent stability and improved output characteristic and low temperature characteristic by including a cathode active material in which at least one of metals forming the cathode active material has a concentration gradient in an entire region from a central portion up to a surface portion; and a conductive material mixture in which carbon nanotube is mixed with carbon black at an appropriate ratio, the carbon black being a spherical nanoparticle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.10-2014-0158679, filed on Nov. 14, 2014, entitled “LITHIUM ION SECONDARYBATTERY”, which is hereby incorporated by reference in its entirety intothis application.

BACKGROUND

1. Technical Field

The present invention relates to a secondary battery, specifically, to asecondary battery having excellent stability and improved outputcharacteristic and low temperature characteristic by including a cathodeactive material in which at least one of metals forming the cathodeactive material has a concentration gradient in an entire region from acentral portion up to a surface portion; and a conductive materialmixture in which carbon nanotube is mixed with carbon black at anappropriate ratio, the carbon black being a spherical nanoparticle.

2. Description of the Related Art

Nowadays, a lithium secondary battery has been widely used in portablemobile devices such as cell phones, and laptop computers. In addition,the lithium secondary battery has received attention as a medium- orlarge-sized battery system for power storage of renewable energy such assolar and wind power, and the like, as well as power sources of hybridelectric vehicles and intelligent robots.

The lithium secondary battery uses a principle in which electricalenergy is generated by a change in chemical potential when lithium ionsare inserted into and desorbed from a cathode and an anode.

The lithium secondary battery is manufactured by including materialscapable of reversibly inserting and desorbing lithium ions as a cathodeactive material and an anode active material, and filling an organicelectrolyte or a polymer electrolyte between the cathode and the anode.

In particular, a lithium composite metal compound has been used as thecathode active material of the lithium secondary battery. For example,composite metal oxides such as LiCoO₂, LiMn2O₄, LiNiO₂,LiNi_(1-x)Co_(x)O₂ (0<x<1), LiMnO₂, LiFePO₄, and the like, have beenresearched so far.

Among the examples, since LiCoO₂ is significantly useful as the cathodeactive material for a secondary battery due to stable charge anddischarge characteristics, excellent electron conductivity, high batteryvoltage, high stability, and flat discharge voltage characteristics, buthas a unstable crystal structure due to lithium elimination duringcharging, which significantly deteriorates thermal characteristics.

In order to overcome the drawback, Korean Laid-Open Publication Nos.2005-0083869, 2007-0097923 suggested a lithium transition metal oxide inwhich metal composition has a concentration gradient, as a cathodeactive material for a secondary battery. However, due to a largeinternal resistance, the lithium transition metal oxide has lower outputcharacteristic than that of the existing cathode active materials.

SUMMARY

It is an aspect of the present invention to provide a secondary batteryhaving excellent stability and improved output characteristic and lowtemperature characteristic by including a cathode active material havinga concentration gradient in an entire region from a central portion upto a surface portion, and further including specific conductivematerials mixed at an appropriate ratio.

Specifically, it is the aspect of the present invention is to provide asecondary battery having excellent stability and improved outputcharacteristic and low temperature characteristic by including a cathodeactive material in which at least one of metals forming the cathodeactive material has a concentration gradient in an entire region from acentral portion up to a surface portion; and a conductive materialmixture in which carbon nanotube is mixed with carbon black which is aspherical nanoparticle, in a cathode mixture used in the secondarybattery of the present invention.

The present invention is not limited to the above aspect and otheraspects of the present invention will be clearly understood by thoseskilled in the art from the following description.

In accordance with one aspect of the present invention, there isprovided a secondary battery having excellent stability and improvedoutput characteristic and low temperature characteristic, the secondarybattery including: a cathode active material in which at least one ofmetals forming the cathode active material has a concentration gradientin an entire region from a central portion up to a surface portion; anda conductive material mixture in which carbon nanotube is mixed withcarbon black which is a spherical nanoparticle.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. It should beunderstood that the present invention is not limited to the followingembodiments, and that the embodiments are provided for illustrativepurposes only. The scope of the invention should be defined only by theaccompanying claims and equivalents thereof.

According to an exemplary embodiment of the present invention, there isprovided a secondary battery having excellent stability and improvedoutput characteristic and low temperature characteristic by including acathode active material in which at least one of metals forming thecathode active material has a concentration gradient in an entire regionfrom a central portion up to a surface portion; and a conductivematerial mixture in which carbon nanotube is mixed with carbon blackwhich is a spherical nanoparticle.

That is, according to the cathode mixture for a secondary batteryaccording to the present invention, the secondary battery havingexcellent stability and improved output characteristic and lowtemperature characteristic may be provided by including a cathode activematerial having a concentration gradient in an entire region from acentral portion up to a surface portion, and further including aconductive material mixture in which specific conductive materials aremixed at an appropriate ratio. To this end, the conductive materialmixture of the present invention may include carbon nanotube and carbonblack.

In order to achieve effect contact between the conductive materials andthe active material to excellently form a conductive path between theactive materials, it is important to select kinds and mixing ratio ofthe conductive materials constituting the conductive material mixture.

First, the conductive material mixture preferably includes carbon blackwhich is a nanoparticle as a first conductive material. The carbon blackhas an approximately spherical shape, and is mixed with carbon nanotube(CNT) as a second conductive material, thereby achieving effectivecontact between the conductive materials and the active material. Theconductive material mixture preferably includes CNT as the secondconductive material.

That is, in the present invention, the first conductive materialincluded in the conductive material mixture has an approximatelyspherical shape, and the second conductive material has a linear shapeformed in a cylinder structure, such that when the first conductivematerial and the second conductive material are employed as componentsof conductive material mixture of the present invention, a poreformation rate is minimized, and accordingly, the conductive pathbetween the conductive materials and the active material may be easilyformed. Accordingly, there is provided a secondary battery havingexcellent stability and improved output characteristic and lowtemperature characteristic.

Here, the first conductive material and the second conductive materialincluded in the conductive material mixture are mixed at a mixing ratioof 10:0.1 to 1:10, particularly, 10:1 to 6:4.

When the mixing ratio of the second conductive material to the firstconductive material is less than 1 wt %, the content of the secondconductive material is not sufficient, such that an effect of improvingthe output characteristic and the low temperature characteristic of thesecondary battery is not exhibited. When the mixing ratio of the secondconductive material to the first conductive material is more than 60 wt%, an excessive amount of the second conductive material increases apore formation rate between the conductive materials and the activematerial, such that a conductive path between the conductive materialsand the active material is not excellently formed, which deterioratesthe output characteristic and the low temperature characteristic of thesecondary battery.

Meanwhile, the cathode active material is represented by ChemicalFormula 1 below, and in Chemical Formula 1, at least one of M1, M2, andM3 has a continuous concentration gradient from the central portion upto the surface portion:

Li_(x)M1_(a)M2_(b)M3_(c)O_(y)  [Chemical Formula 1]

in Chemical Formula 1,

M1, M2 and M3 are selected from the group consisting of Ni, Co, Mn, Na,Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga and B, and

0<x≦1.1, 2≦y≦2.02, 0≦a≦1, 0≦b≦1, 0≦c≦1, 0<a+b+c≦1.

That is, the cathode active material of the present invention has aconcentration gradient in which a concentration of some of metals iscontinuous in the entire region from the central portion up to thesurface portion of the particle, unlike the conventional cathode activematerials having a concentration gradient in which a concentration ofmetals is uniform in an internal area of the cathode active material andthe concentration of metals is gradual only in an external area thereof.

Specifically, at least one of M1, M2, and M3 may have a concentrationgradient region in which a concentration is increased from the centralportion up to the surface portion, and the remainder may have aconcentration gradient region in which a concentration is decreased fromthe central portion up to the surface portion.

Preferably, in the cathode active material of the present invention, aconcentration of one of metals forming the cathode active material isuniform in the entire region from the central portion up to the surfaceportion of the particle, and concentrations of other two metals areincreased or decreased in the entire region from the central portion upto the surface portion of the particle, respectively, while having acontinuous concentration gradient.

Any one of M1, M2, and M3 may have a predetermined concentration fromthe central portion up to the surface portion, and another one may havea concentration gradient region in which a concentration is increasedfrom the central portion up to the surface portion, and the other onemay have a concentration gradient region in which a concentration isdecreased from the central portion up to the surface portion.

Specifically, M1 may be Ni, M2 may be Mn, and M3 may be Co.

In the present invention, the metal of the lithium-metal oxide has thecontinuous concentration gradient from the central portion up to thesurface portion, which means that a metal except for lithium has aconcentration distribution in which concentration is changed in acertain tendency from the central portion up to the surface portion ofthe lithium-metal oxide particle. The certain tendency means thatoverall concentration change trend is decreased or increased, but doesnot exclude values at some points being opposite to this trend. In thepresent invention, the central portion of the particle means a portionwithin a radius of 0.2 μm from the center of the active materialparticle, and the surface portion of the particle means a portion withina radius of 0.2 μm from the outermost of the particle.

Preferably, the slope of the concentration gradient in the presentinvention is uniform from the central portion up to the surface portionof the particle, that is, the concentration is uniformly changed.

The cathode active material particle according to the present inventionmay include a relatively large content of nickel (Ni). Nickel is used toimprove battery capacity. In the conventional cathode active materialstructure, when the content of nickel is large, lifespan isdeteriorated. However, in the case of the cathode active materialaccording to the present invention, even though the content of nickel islarge, lifespan is not deteriorated. Accordingly, the cathode activematerial of the present invention may exhibit excellent lifespancharacteristic while maintaining high capacity.

For example, in the cathode active material particle according to thepresent invention, a molar ratio of nickel may be 0.6 to 0.9,preferably, 0.7 to 0.9. That is, when M1 is Ni in Chemical Formula 1above, 0.6≦a≦0.95 and 0.05≦b+c≦0.4, preferably, 0.7≦a≦0.9 and0.1≦b+c≦0.3, may be satisfied.

The cathode active material particle according to the present inventionis not specifically limited in view of a particle shape, but preferably,a primary particle may have a rod-type shape.

The cathode active material particle according to the present inventionis not specifically limited in view of a particle size. For example, thecathode active material particle may have a particle size of 3 to 20 μm.

Meanwhile, the content of the cathode active material and the conductivematerial mixture in the present invention is preferably 2 to 8 parts byweight (wt %), more preferably, 3 to 5 parts by weight (wt %) relativeto 100 parts by weight (wt %) of the cathode active material.

The present invention provides a cathode including the cathode activematerial and the conductive material mixture, wherein the cathode may beeffectively used for a cathode of an electrochemical cell such as asecondary battery. The secondary battery includes an anode including ananode active material and an electrolyte, together with the cathode.

The cathode includes a current collector and a cathode layer formed onthe current collector.

The cathode may include a binder in addition to the above-describedcathode and the conductive material mixture. The binder serves toexcellently attach cathode active material particles to each other, andexcellently attach the cathode active material to a current collector.Representative examples of the binder may include polyvinyl alcohol,carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose,polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride,a polymer including ethylene oxide, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and the like. However,the present invention is not limited to the above-described binders.

The current collector may be aluminum (Al), but the present invention isnot limited thereto.

The anode includes a current collector and an anode active materiallayer formed on the current collector, wherein the anode active materiallayer includes an anode active material. The anode active materialincludes a material capable of reversibly inserting and desorbinglithium ions, a material capable of doping and de-doping lithium metals,lithium metal alloys, lithium, or a transition metal oxide.

The material capable of reversibly inserting and desorbing lithium ionsis a carbon material, wherein the carbon material may be anycarbon-based anode active material generally used in a lithium ionsecondary battery, and representative examples of the carbon-based anodeactive material may include crystalline carbon, amorphous carbon, orcombinations thereof. Examples of the crystalline carbon may includeamorphous, plate-shaped, flaky, spherical or fibrous natural graphite orartificial graphite. Examples of the amorphous carbon may include softcarbon (low temperature fired carbon), hard carbon, mesophase pitchcarbide, fired cokes, and the like.

The lithium metal alloys may be alloys of lithium with a metal such asNa, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al orSn.

The material capable of doping and de-doping lithium may be Si, SiO_(x)(0<x<2), Si-M alloy (wherein M is an alkali metal, an alkaline earthmetal, Group 13 to 16 element, a transition metal, a rare earth elementor combinations thereof, excluding Si), Sn, SnO₂, Sn-M (wherein M is analkali metal, an alkaline earth metal, Group 13 to 16 element, atransition metal, a rare earth element or combinations thereof,excluding Si), and the like, and further, may be used in combination ofat least one thereof and SiO₂. The element M may be Mg, Ca, Sr, Ba, Ra,Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb,Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti,Ge, P, As, Sb, Bi, S, Se, Te, Po or combinations thereof.

The transition metal oxide may include vanadium oxide, lithium vanadiumoxide, and the like.

The anode active material layer may also include a binder, andselectively, may further include a conductive material.

The binder serves to excellently attach anode active material particlesto each other, and excellently attach the anode active material to acurrent collector. Representative examples of the binder may includepolyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose,polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride,a polymer including ethylene oxide, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and the like. However,the present invention is not limited to the above-described binders.

The conductive material is used to provide conductivity to theelectrodes. As the conductive material, any electro-conductive materialmay be used as long as chemical changes do not occur in a battery to beconfigured. Examples of the conductive material may include carbon-basedmaterials such as natural graphite, artificial graphite, carbon black,acetylene black, Ketjen black, carbon fibers, and the like; metal-basedmaterials such as metal powder and metal fibers, including copper,nickel, aluminum, silver, and the like; conductive polymers such aspolyphenylene derivatives, and the like; or conductive materialsincluding mixtures thereof.

The collector may be a copper foil, a nickel foil, a stainless steelfoil, a titanium foil, a nickel foam, a copper foam, a polymer substratecoated with conductive metals, or combinations thereof.

The cathode and the anode are manufactured by mixing each activematerial, conductive material, and a binder in a solvent to prepare eachactive material composition, and applying the composition on a currentcollector. Since the methods of manufacturing electrodes as describedabove are widely known in the art, detailed description thereof isomitted in the present invention. The solvent may beN-methylpyrrolidone, and the like, but the present invention is notlimited thereto.

The electrolyte charged in the lithium secondary battery may be anon-aqueous electrolyte, a known solid electrolyte, or the like, or maycontain lithium salt dissolved therein. Examples of solvents of thenon-aqueous electrolyte may include cyclic carbonates such as ethylenecarbonate, diethylene carbonate, propylene carbonate, butylenecarbonate, vinylene carbonate, and the like; chain carbonates such asdimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and thelike; esters such as methyl acetate, ethyl acetate, propyl acetate,methyl propionate, ethyl propionate, γ-butyrolactone, and the like;ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran,1,2-dioxane, 2-methyltetrahydrofuran, and the like; nitriles such asacetonitrile, and the like, amides such as dimethylformamide, and thelike. However, the present invention is not limited to theabove-described solvents of the non-aqueous electrolyte. The solvent ofthe non-aqueous electrolyte may be used alone or in combination of aplurality of these solvents. In particular, a mixed solvent in which thecyclic carbonate and the chain carbonate are mixed, may be used.

In addition, the electrolyte may be a gel polymer electrolyte in which apolymeric electrolyte such as polyethylene oxide, polyacrylonitrile, orthe like, impregnated with an electrolytic solution or may be aninorganic solid electrolyte such as LiI, Li₃N, or the like. However, thepresent invention is not limited to the above-described electrolytes.

Here, the lithium salt may be selected from the group consisting ofLiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃,LiSbF₆, LiAlO₄, LiAlO₂, LiAlCl₄, LiCl and LiI. However, the presentinvention is not limited to the above-described lithium salts.

A separator may be present between the cathode and the anode dependingon types of a lithium secondary battery. The separator may be a singlelayer film of polyethylene, polypropylene, polyvinylidene fluoride, or amultilayer film having two or more layers thereof. In addition, theseparator may be a complex multilayer film, such as a two-layeredseparator of polyethylene/polypropylene, a three-layered separator ofpolyethylene/polypropylene/polyethylene, or a three-layered separator ofpolypropylene/polyethylene/polypropylene.

The lithium secondary battery may be classified into a lithium ionbattery, a lithium ion polymer battery, and a lithium polymer batteryaccording to kinds of the separator and the electrolyte to be used, ormay be classified into a cylindrical shape, a prismatic shape, acoin-type shape, a pouch-type shape, and the like, according to shape,or may be classified into a bulk type and a thin film type according tosize. Since structures of these batteries and methods of manufacturingthe batteries are widely known in the art, detailed description thereofis omitted in the present invention.

Hereinafter, a configuration and a method for implementing objects ofthe present invention are described through the following Examples inmore detail. However, the scope of the present invention is not limitedto these Examples.

Examples 1 to 8

(1) Cathode

Each cathode was manufactured by including lithium-metal oxide(LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂) having a concentration gradient from acentral portion (LiNi_(0.84)Co_(0.11)Mn_(0.05)O₂) up to a surfaceportion (LiNi_(0.78)Co_(0.10)Mn_(0.12)O₂) (hereinafter, referred to asCAM-10) as a cathode active material, a mixture of denca black andcarbon nanotube (see mixing ratios of Table 2) as conductive materials,and PVDF as a binder, at a mass ratio of 92:5:3. Then, each cathode wascoated, dried, and pressed on an aluminum substrate, thereby completingeach final cathode.

Concentration gradient of each cathode active material is shown in Table1 above. For concentration measurement, concentrations of the cathodeactive material particle were measured at an interval of 5/7 μm from thesurface, the cathode active material particle having a distance from thecenter of the cathode active material up to a surface thereof of 5 μm.

TABLE 1 Position Ni (wt %) Co (w %) Mn (wt %) Surface Portion 77.9710.07 11.96 Concentration Gradient 80.98 9.73 9.29 82.68 10.32 7 82.6 107.4 82.55 10.37 7.07 83.24 10.86 5.9 Central Portion 84.33 10.83 4.84

(2) Anode

Each anode was manufactured by preparing an anode mixture including 93wt % of natural graphite as an anode active material, 5 wt % of KS6which is a flake type conductive material as a conductive material, 1 wt% of SBR as a binder, and 1 wt % of CMC as a thickener, and coating,drying, and pressing the prepared anode mixture on a copper substrate.

(3) Battery

Each battery was manufactured by stacking a cathode plate and an anodeplate each having an appropriate size and being notched and interposinga separator (polyethylene having a thickness of 25 μm) between thecathode plate and the anode plate. Then, welding was performed on eachtab portion of the cathode and the anode. The weldedcathode/separator/anode was put into a pouch, and sealing was performedon three sides except for a side in which the electrolyte is injected.Here, the tap portions were included in the sealing portions. Theelectrolyte was injected into the remaining side, and the remaining sidewas sealed, followed by impregnation for 12 hours or more. Theelectrolyte was prepared by making 1M LiPF₆ solution with a mixedsolvent of EC/EMC/DEC (25/45/30 by volume), and adding 1 wt % ofvinylene carbonate (VC), 0.5 wt % of 1,3-propene sultone (PRS), and 0.5wt % of lithium bis(oxalate)borate (LiBOB) thereto.

Then, pre-charging was performed at a current (2.5 A) corresponding to0.25C for 36 minutes. After 1 hour, degassing was performed, followed byaging for 24 hours or more, and formation charging and discharging(charging condition CC-CV 0.2C 4.2V 0.05C CUT-OFF, discharging conditionCC 0.2C 2.5V CUT-OFF). Then, standard charging and discharging wasperformed (charging condition CC-CV 0.5 C 4.2V 0.05C CUT-OFF,discharging condition CC 0.5C 2.5V CUT-OFF).

Comparative Examples 1 to 8

Each battery was manufactured by the same method as Examples 1 to 8except for using LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ (hereinafter, referred toas NCM811) having uniform composition in the entire particle, instead ofusing CAM-10, as the cathode active material.

Comparative Examples 9 to 12

Each battery was manufactured by the same method as Example 1 except forusing each cathode active material and each conductive material asdescribed in Table 2 below.

Evaluation

1. Output Characteristic

Output characteristic of each battery manufactured by Examples andComparative Examples was measured by HPPC (Hybrid Pulse PowerCharacterization) test by Freedom Car Battery Test Manual. Resultsthereof were shown in Table 2 below.

2. Low Temperature Characteristic (Discharge Capacity at −20° C. AsCompared to Discharge Capacity at Room Temperature)

Low temperature characteristic of each battery manufactured by Examplesand Comparative Examples was measured by comparison between dischargecapacity at room temperature (25° C.) and discharge capacity at −20° C.(0.5C charged and 0.5C discharged). Results thereof were shown in Table2 below.

TABLE 2 Low Temperature Characteristic (discharge capacity at −20° C. ascompared Conductive Conductive to discharge capacity Cathode ActiveConductive Material 1 Conductive Material 2 Output at room temperature)Material Material 1 (wt %) Material 2 (wt %) (W/kg) (%) ComparativeNCM811 Denca black 4.9 CNT 0.1 3000 70 Example 1 Comparative NCM811Denca black 4.75 CNT 0.25 3025 73 Example 2 Comparative NCM811 Dencablack 4.5 CNT 0.5 3050 75 Example 3 Comparative NCM811 Denca black 4 CNT1 3100 78 Example 4 Comparative NCM811 Denca black 3 CNT 2 3200 80Example 5 Comparative NCM811 Denca black 2 CNT 3 3200 79 Example 6Comparative NCM811 Denca black 1 CNT 4 3150 75 Example 7 ComparativeNCM811 Denca black 0.1 CNT 4.9 3100 72 Example 8 Example 1 CAM-10 Dencablack 4.9 CNT 0.1 2800 60 Example 2 CAM-10 Denca black 4.75 CNT 0.253100 65 Example 3 CAM-10 Denca black 4.5 CNT 0.5 3120 72 Example 4CAM-10 Denca black 4 CNT 1 3200 76 Example 5 CAM-10 Denca black 3 CNT 23300 80 Example 6 CAM-10 Denca black 2 CNT 3 3300 75 Example 7 CAM-10Denca black 1 CNT 4 3150 70 Example 8 CAM-10 Denca black 0.1 CNT 4.93100 65

It could be appreciated from Table 2 that the batteries of Examples hadexcellent output characteristic and low temperature characteristic(discharge capacity at −20° C. as compared to discharge capacity at roomtemperature) as compared to the batteries of Comparative Examples.

Specifically, it could be confirmed that in the cathode active materialhaving the concentration gradient according to the present invention andthe predetermined mixing ratio of denca black and carbon nanotube (CNT)(ranging from 4.75:0.25 to 3:2), the batteries of Examples hadrelatively excellent output characteristic and low temperaturecharacteristic as compared to the batteries of Comparative Examples.Specifically, it could be appreciated that as the content of CNT in theratio of denca black and CNT increased, the batteries of ComparativeExamples had output characteristic improved by about 200 Wh/kg(3000→3200 Wh/kg); meanwhile, the batteries of Examples had outputcharacteristic improved by about 400 Wh/kg (2800→3200 Wh/kg), such thatthe increase in output characteristic of Examples was larger than thatof Comparative Examples. In addition, it could be confirmed thatimprovement extent in view of low temperature characteristic of Exampleswas also larger than that of Comparative Examples. When the mixing ratioof denca black and carbon nanotube (CNT) was over than a predeterminedratio (a case in which the mixing ratio was over 3:2 in the presentresult), the output characteristic and the low temperaturecharacteristic were rather deteriorated, which is resulted from failureof effective contact between the conductive materials and the activematerial. It may be appreciated that when the tube-shaped CNT and dencablack which is a spherical nanoparticle are mixed at an appropriateratio according to the present invention, connection between the activematerials are excellently achieved to favorably form a conductive path,wherein if any one of CNT and denca black has an excessively largercontent than the other one, a relatively large amount of pores occur,which deteriorates conductivity.

According to the cathode mixture for a secondary battery according tothe present invention, the secondary battery having excellent stabilityand improved output characteristic and low temperature characteristicmay be provided by including a cathode active material having aconcentration gradient in an entire region from a central portion up toa surface portion, and further including specific conductive materialsmixed at an appropriate ratio.

That is, the specific conductive materials mixed at an appropriate ratioare also used with the cathode active material, such that a conductivepath between the cathode active material and the conductive materialsmay be smoothly formed to improve output characteristic and lowtemperature characteristic.

The above description of the present invention is provided forillustrative purposes, and it will be understood to those skilled in theart that the exemplary embodiments can be easily modified into variousforms without changing the technical spirit or essential features of thepresent invention. Accordingly, the exemplary embodiments describedherein are provided by way of example only in all aspects and should notbe construed as being limited thereto.

What is claimed is:
 1. A secondary battery comprising: a cathode activematerial in which at least one of metals forming the cathode activematerial has a concentration gradient in an entire region from a centralportion up to a surface portion; and a conductive material mixture inwhich carbon nanotube is mixed with carbon black.
 2. The secondarybattery of claim 1, wherein the carbon black is a sphericalnanoparticle.
 3. The secondary battery of claim 1, wherein the carbonnanotube and the carbon black are mixed at a mixing ratio of 10:0.1 to1:10.
 4. The secondary battery of claim 1, wherein the carbon nanotubeand the carbon black are mixed at a mixing ratio of 10:1 to 6:4.
 5. Thesecondary battery of claim 1, wherein the cathode active material isrepresented by Chemical Formula 1 below, and in Chemical Formula 1, atleast one of M1, M2, and M3 has a continuous concentration gradient fromthe central portion up to the surface portion:Li_(x)M1_(a)M2_(b)M3_(c)O_(y)  [Chemical Formula 1] in Chemical Formula1, M1, M2 and M3 are selected from the group consisting of Ni, Co, Mn,Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga and B,and 0<x≦1.1, 2≦y≦2.02, 0≦a≦1, 0≦b≦1, 0≦c≦1, 0<a+b+c≦1.
 6. The secondarybattery of claim 5, wherein at least one of M1, M2, and M3 has aconcentration gradient region in which a concentration is increased fromthe central portion up to the surface portion, and the remainder has aconcentration gradient region in which a concentration is decreased fromthe central portion up to the surface portion.
 7. The secondary batteryof claim 5, wherein any one of M1, M2, and M3 has a predeterminedconcentration from the central portion up to the surface portion, andanother one has a concentration gradient region in which a concentrationis increased from the central portion up to the surface portion, and theother one has a concentration gradient region in which a concentrationis decreased from the central portion up to the surface portion.
 8. Thesecondary battery of claim 7, wherein M1 is Ni, M2 is Mn, and M3 is Co.9. The secondary battery of claim 5, wherein M1 is Ni, and 0.6≦a≦0.95and 0.05≦b+c≦0.4 are satisfied.
 10. The secondary battery of claim 5,wherein M1 is Ni, and 0.7≦a≦0.9 and 0.1≦b+c≦0.3 are satisfied.
 11. Thesecondary battery of claim 1, wherein the conductive material mixturehas a content of 2 to 8 wt % relative to 100 wt % of the cathode activematerial.